Three women who were freed from a London home after 30 years had been allowed outside in "carefully controlled circumstances" during their ordeal but were victims of "slavery, in simple terms", a senior British police officer says.

Commander Steve Rodhouse described a "complicated and disturbing picture of emotional control over many years" in the case of the women, declining to say how they wound up in the south London home. Two suspects, a man and a woman, were arrested early on Thursday (local time) on suspicion of forced labour and domestic servitude.

He said investigators are trying to figure out "what were the invisible handcuffs that were used" to exert such control for the 30 years the women were allegedly held captive and subject to physical, mental and emotional abuse.

"It is not as brutally obvious as women being physically restrained inside an address and not being allowed to leave," Rodhouse said. "This may have appeared to be a normal family."

The disclosure Thursday that a 69-year-old Malaysian, a 57-year-old Irish woman and a 30-year-old Briton were freed after apparently spending 30 years in captivity prompted a flurry of speculation and questions about how such a tragedy escaped notice for so long.

The arrests were made after the Irish woman phoned a charity last month to say she was being held against her will along with two others. The charity engaged in a series of secretive conversations with the women and contacted police. Two of the women eventually left the house, and police rescued the third.

The case has sent shockwaves throughout Britain and around the world, but is the latest horrifying case of a broader phenomenon that officials warn is still happening - and on the rise.

"Cases of modern slavery are becoming more prevalent in Europe," said Rob Wainwright, director of Europol, the European Union's law enforcement agency. "Unfortunately, it remains a low priority for many national police authorities. Europol is committed to fostering stronger international police action in this area and to raising greater levels of public attention."

Since the most recent expansions of the EU and the lifting of restrictions on employment in many countries, instances of situations which amount to forced labour have increased, Europol says.

Anti-slavery charity The Walk Free Foundation last month released a global index that estimated that more than 29 million people live in some form of modern slavery - which can take the form of domestic servitude, forced marriages, child trafficking and forced labour.

While the index found that Africa and Asia are home to the vast majority of modern slaves, it estimated that there are up to 4600 slaves in the UK - a country which had the lowest estimated prevalence of slavery in a ranking of 162 countries.

Fiona David, of Walk Free, said on Friday that the most recent case in London highlights that slavery can be perpetrated by "just people living in the neighbourhood", not necessarily "organised criminals".

The suspects - both 67 - have also been questioned on suspicion of immigration offences, police said. Regarding their identity, police would only say that the suspects were not British and had been in the country for "many years". They would not elaborate on the pair's nationalities, but did note that both had been arrested in the 1970s. Police did not say why.

Both suspects have been released on bail, having surrendered their passports and agreed not to return to their house as part of their bail conditions.

Police say they do not believe there are other victims in the case, and they are confident that they know the true identities of the three women. The relationships among the women - and between the women and the suspects - are under investigation, police said.

"Whilst we do not believe that they have been subjected to sexual abuse, we know that there has been physical abuse, described as beatings," said Kevin Hyland, head of the Metropolitan Police's human trafficking unit.

He and Rodhouse defended the lapse in time between October 25 - when the women gained freedom - and the arrests, saying that the case is complicated and investigators must be careful to ensure they do not further traumatise the victims as they unravel the events of the past 30 years.

Hyland also urged patience as the case unfolds, noting that officers must sift through 55 bags of evidence seized in a search of the home, interview several people and follow up numerous strands.

Police noted that the case was "unique" not least because of the length of women's ordeal, though the UK has seen a string of high-profile slavery cases in recent years.

Last month, an 84-year-old man was jailed for repeatedly raping a deaf and mute girl he trafficked into Britain from Pakistan and kept as a virtual slave.

Police in Wales have arrested at least seven people in recent months as part of a long-running investigation into a suspected slavery ring there.